Transformer having plural part primary and secondary windings



July.30, 1968 J. T. STOREY 3,395,335

TRANSFORMER HAVING PLURAL PART PRIMARY AND SECONDARY WINDINGS Filed Oct. 7, 1966 FIG/l,

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. F l G 2 PRIOR ART F l 3 P2 P1 P2 P3 5 INVENTOR BY JOHNTHOMAS STOREY WWZUM ATTORNEYS United States Patent 3,395,335 TRANSFORMER HAVING PLURAL PART PRIMARY AND SECONDARY WINDINGS John Thomas Storey, Greenford, England, assignor to Haddon Transformers Limited, Rnislip, England Filed Get. 7, 1966, Ser. No. 585,089 3 Claims. (Cl. 323-435) ABSTRACT on THE DISCLOSURE A press package transformer for producing a plurality of spot welds simultaneously comprising N primary windings and N-l single-turn secondary windings arranged alternately with the primary windings, an intermediate primary winding being connected to one terminal of an AC. source while the two end primaries are connected in parallel through primary taps with the other terminal of the AC. source.

This invention relates to transformers and particularly to those commonly known in the trade as press package transformers and more rarely has multi-press type transformers such as are installed in multi-press welding equipment and used, particularly in the motor industry, to perform simultaneously a plurality of resistance spot welds in a single assembly and operation.

At the present time these transformers are single phase units but it should be understood that the arrangement according to the present invention is applicable to polyphase transformers. 'In the single phase units the primary windings are connected through switching means to a mains supply, and comprise N primary windings and N-1 secondary windings. For up to 200 kva. ratings, N=3, but for ratings in excess of 200 kva., N may be greater than 3.

' The secondary comprises two equal windings, each with open circuit voltages of up to 25 volts and capable of delivering many thousands of amperes, and each secondary winding is arranged to pass cooling water therethrough or therearound. The windings of the transformer are located within an enclosing electromagnetic core with each secondary winding located intermediate a pair of primary windings, i.e., the primary and secondary windings are arranged alternately with two of the primary windings constituting the end windings of the assembly,

and the adjacent windings are separated by electrically insulating members.

Heretofore, the primary end windings have been connected in series with each other and across the mains input; one terminal of the source being connected via a switch, to one end of one of the series connected windings, such that the two primary end windings are always in circuit across the source. With such an arrangement, the primary winding taps are taken from the, or an inter- .mediate primary winding and must necessarily come between the tapped intermediate primary winding and one of the adjacent secondary windings. Consequently, the spacing between the tapped intermediate primary winding and the adjacent secondary winding between which the primary winding taps are taken, is different from the spacing between the tapped primary and the other adjacent secondary winding.

Since the leakage reactance between windings of a transformer increases with the distance between the windings, the leakage reactance between the tapped intermedihighest voltage, the output currents from the two secondary windings adjacent the tapped primary, are different.

Accordingly, it is an object of the present invention to provide a transformer wherein the leakage reactance between a tapped primary winding and its adjacent secondary windings is uniform.

A further object of the present invention is to provide a press package transformer wherein the leakage reactance between a tapped primary winding and its adjacent secondary winding is lower than that of press package transformers heretofore used.

A still further object of the present invention is to provide a water cooled press package transformer wherein the thermal transfer from primary to secondary windings is greater than with press package transformers known heretofore with a corresponding increase in the efiiciency thereof.

Thus, according to the present invention there is provided a press package transformer having N primary windings and N-l secondary windings arranged alternately with the primary windings, two of which form the end windings of the assembly, electrically insulating memhers being located between adjacent windings and at each end of the assembly, said end primary windings being connected in parallel to an AC. source and primary taps being taken from said end primary windings and passing out through the end insulating members of the assembly such that the spacing between windings is uniform, and is reduced to the minimum necessary for adequate thickness of insulation to obtain uniform leakage reactance between the secondary and primary windings and a reduced leakage reactance compared with that of a transformer having the primary taps taken from an intermediate primary winding of the assembly.

With the arrangement according to the present invention, each secondary winding has two coupling surfaces between it and its associated primary winding and a lower leakage reactance with a corresponding increase in the secondary output is obtained.

The reduction of the spacing between windings enables greater thermal transfer to be effected therebetween with improved efliciency.

The constructional features of the present invention will now be described by way of example with particular reference to the accompanying drawing where a transformer constructed in accordance with the present invention is compared with a prior known construction to illustrate the advantages of the present invention.

In the drawing:

FIGURE 1 illustrates the geometrical relationship between the primary and secondary windings of a press package transformer of up to 200 kva. rating, i.e., where FIGURE 2 illustrates the method of connection of the primary windings and the primary overwind taps used in prior known press package transformers of the rating referred to above; and

FIGURE 3 illustrates the method of connection of the primary windings and the primary overwind taps according to the present invention.

Referring firstly to FIGURE 1, the transformer includes N primary windings P1, P2, P3, with N-l secondary windings S1, S2, with N=3. The windings are located within an enclosing electromagnetic core K and windings P1, P2, P3 are of the pancake type and S1, S2, are secondary single turn windings. The primary and secondary windings are assembled in alternate relationship with primary windings P1 and P3 constituting the end windings of the assembly. Electrically insulating members, A, B, C, D, E. F. G and H are located intermediate the primary and secondary windings and surrounds the assembly as shown.

The primary coils are preferably wound with copper tape of almost the same width as the winding, the turns being placed one over the other and insulated from each other by a flexible insulating tape such as Woven cotton or fibre glass.

The tappings are brought out by other copper tapes, which are soldered, welded or brazed at right angles to the winding copper tape and brought up the side f the coil.

The single turn secondary windings may each be cast from high conductivity copper in which case a copper tube is welded, soldered or brazed into, and flush with, the side of the casting, for the purpose of carrying cooling fluid. Alternatively, the secondary windings may each be formed from extruded high conductivity copper tubing having special lugs welded, soldered or brazed on to the ends thereof. In this case, cooling fluid passes through the secondary windings.

Referring now to FIGURE 2 of the drawings there is illustrated a prior known method of connection of the primary windings and taps. Primary windings P1 and P3 are connected in series with each other across the mains supply and with the switch S in the position shown, it will be seen that end primary windings P1 and P3 are always in circuit, whilst the primary taps are all taken from intermediate primary winding P2. With such an arrangement, the tapping leads must necessarily come between tapped intermediate primary windings P2 and one of the adjacent secondary windings S1 or S2. Accordingly, the spacing between windings P2 and S1 is different from that between windings P2 and S2. It is well known that the leakage reactance between windings of a transformer is proportional to the distance between the windings. Thus, with the arrangement of FIGURE 2, the leakage reactance between windings P2 and S1 is different from that between windings P2 and S2. This r sults in the output currents from secondary windings S1 and S2, being different, when the transformer is loaded with two identical loads, and when it is operating on secondary voltages less than the highest output value; the leakage reactance between windings P2 and S1 and 52. not being in circuit on the highest output, i.e., when the switch S is in the position shown in FIGURE 2.

Furthermore, with the arrangement shown in FIGURE 2, on the highest output, each secondary winding has only one coupling face to the primary winding. Since the leak age inductance in a transformer is inversely proportional to the number of coupling faces between the windings, it will be appreciated that if the coupling faces between windings can be increased, the leakage reactance between windings can be reduced and a greater secondary output achieved. Also if the insulation and spacing between windings can be reduced, a greater thermal transfer may be efiected through the cooling fluid associated with the secondary windings, resulting in a lower temperature rise of the transformer when on load.

Thus, if the spacing between windings can be made uniform and reduced and the coupling faces increased, the leakage reactance between windings can be made uniform giving equal output currents for identical loads, the leakage reactance reduced to give greater secondary output and thermal transfer increased to give a lower temperature rise with increased efficiency.

The arrangement shown in FIGURE 3 achieves the above results and overcomes the disadvantages of the prior known construction of FIGURE 2.

Referring to FIGURE 3, the end primary coils P1 and P3 are paralleled and this arrangement is connected in series with primary coil P2 across the mains supply. With such an arrangement, the primary taps can be brought out through the end insulation member A and C of the assembly. This means that the spacing between windings P2 and S1 and S2 can be made uniform and the insulation members F and G (FIGURE 1) reduced in thickness to the lowest practicable minimum, resulting in unip :3. form and reduced leakage reactance between windings with consequent equal output currents withddentical loads and increased thermal efficiency.

Furthermore, each secondary winding has two coupling faces between it and the :primary, compared with the one coupling face to the primary of the arrangement of FIG- URE 2. This will result in a lower leakage reactance with a corresponding increase in the secondary output current.

It will be seen from FIGURE 3 that intermediate primary winding P2 is not tapped and for the highest secondary output, i.e., with the switch in the position shown, the circuit includes all the turns of primary winding P2 together with part of winding P1 and part of winding P3 connected in paiallel.

Considering the other tapping points of the arrangement shown in FIGURE 3," it will be seen that anequal balance of leakage reactance exists between the primaries and each'secondary winding. I 1

By the reduction of the thicknessof the insulating members E, F, G and H (FIGURE 1) to the minimum practicable value, the overall leakage inductance of the transformer is reduced. To retain the uniformity of spacing between windings, the start lead of primary winding P2 passes under windings S1 and P1 and emerges from the coil assembly at the junction of insulating members A and B (FIGURE 1), The advantage of reducing the thickness of the insulation members F and G, to a-minimum, is that either more active material may be used in place thereof, thus increasing the possible output of the transformer for the same overall size, or alternatively,

the size and cost of the transformer may be reduced,' for' the same output.

It will be appreciated that a typical resistance welding load is inductive; the usual values of power factor lying between 0.4 and 0.8. Thus, any reduction of the leakage reactance of a resistance welding transformer would produce a corresponding greater increase in the electrical efficiency of the transformer, than would be the case with a resistive load.

As previously indicated, the secondaries are cooled by fluid passing therearound or therethr-ough, and thus the secondaries act as heat sinks. Accordingly, the greater the thermal transfer from primary to secondary, the greater the efliciency of the cooling system of the transformer. Since electrical insulation has a poor thermal transfer value, the reduction of electrical insulation between primary and secondary windingsimproves the thermal transfer and results in a lower temperature rise of the transformer when on load. The following table illustrates the difference between the two methods of construction illustrated in FIGURES 2 and 3; the results quoted belowhaving been. obtained between two transformers of identical size and output voltage.

Construction of Construction of I Figure 2 Figure 3 Efficiency, percent 35 46 Output Current, amperes. 17, 900 23,900 Temperature Rise, "0 35 Balance between secondary output 8 2 0 4 (worst condition), percent bers being located between adjacent windings and at each end of the assemblyfsaid end primary windings being connected in'parallel to an AC. sourceand primary taps being taken'from' said end primary windings passing out through the end insulating members of the assembly" such that the spacing between windings is uniform and is reduced to the minimum necessary for adequate thickness of insulation to obtain uniform leakage reactance between the secondary and .primary windings and a reduced leakage reactance compared with that of a transformer having the primary wind taips taken from an intermediate primary winding of the assembly.

2. A press package transformer as claimed in claim 1 where N =3.

3. A press package transformer as claimed in claim 2 wherein the end primary windings are connected in parallel with one another and in series with the intermediate :primary winding across the AL. source.

6 References Cited UNITED STATES PATENTS 0 LEWIS H. MYERS, Primary Examiner.

T. I. KOZMA, Assistant Examiner. 

